Why does my JavaScript code receive a "No 'Access-Control-Allow-Origin' header is present on the requested resource" error, while Postman does not?

If you have worked with JavaScript and made requests to an external API, you may have encountered the error message "No 'Access-Control-Allow-Origin' header is present on the requested resource." This error occurs due to the browser's same-origin policy, which restricts JavaScript requests made from one domain to another domain. However, you may wonder why this error occurs when making requests via JavaScript, while tools like Postman do not encounter the same issue. In this article, we will explore the reasons behind this error and understand why it occurs. We will also discuss possible solutions to overcome this error and make successful cross-origin requests using JavaScript.

Understanding the Same-Origin Policy

The same-origin policy is a security mechanism implemented by web browsers to prevent malicious scripts from accessing data from other domains. It ensures that JavaScript running on a web page can only make requests to the same domain from where it originated.

What is Cross-Origin Resource Sharing (CORS)?

Cross-Origin Resource Sharing (CORS) is a mechanism that allows servers to specify who can access their resources. It is an HTTP header-based system that enables cross-domain communication from web browsers.

Why Does Postman Work Without CORS Issues?

Postman, as a standalone application, does not enforce the same-origin policy. It allows unrestricted requests from any domain, so you won't encounter CORS-related issues. The absence of CORS restrictions in Postman helps developers test APIs without worrying about cross-origin limitations.

Why Does JavaScript Code Encounter CORS Issues?

When making requests using JavaScript, the browser enforces the same-origin policy. This policy restricts JavaScript requests from being made across different domains without proper authorization. Any request made from one domain to another domain violates the same-origin policy unless the requested resource explicitly allows it through CORS.

How CORS Works

When a browser makes a cross-origin request using JavaScript XMLHTTPRequest or the fetch API, it first sends a preflight request with the HTTP OPTIONS method to check if the server allows the actual request. The server, if configured properly, responds with the appropriate Access-Control-Allow-Origin header, allowing the browser to proceed with the actual request.

How to Enable CORS on the Server

If you are developing the API or have control over the server, you can enable CORS by setting the appropriate response headers. The most commonly used header is Access-Control-Allow-Origin, which specifies the allowed domains to make cross-origin requests. For example:

In this example, the server allows cross-origin requests only from the domain "https://example.com". To allow requests from any domain, you can set the header value to "*". However, allowing requests from any domain is generally not recommended due to security concerns.

Handling CORS Issues Without Server Control

If you do not have control over the server or cannot modify the server's response headers, you can use alternative methods to bypass the CORS restrictions:

1. Proxy Server

One approach is to set up a proxy server that makes the actual request on behalf of the JavaScript code. This way, the request is made from the same domain, and hence, it does not violate the same-origin policy. The proxy server acts as an intermediate server between the browser and the API server, forwarding the request and returning the response.

2. JSONP (JSON with Padding)

JSONP is a workaround for making cross-domain requests that bypass CORS restrictions. It involves dynamically adding a